The front wheel offset is only about 1/8" different from the 18 x 8.5 wheel that Ford used on the 2008 GT (+45). The outer face of the wheel will sit that much further in, but you can recover that (plus a tiny bit) appearance-wise with 255/35-20 front tires.

The rear wheel offset is only about 1/4" different from the 2008 GT500 wheel (+45), and the outer face of the wheel will be about 1/2" further out than the GT500's wheel with 1/4" being due to your extra 1/2" of width. All things considered, the best tire match for the above front tires is probably 295/30-20. 305/30's are too much tire for 10" rims and 305/35's are probably too tall and might create ABS difficulties.